1. Field of the Invention
The present invention relates to a road-surface friction-coefficient estimating device of a vehicle estimating a road-surface friction coefficient over a broad driving range.
2. Description of the Related Art
In recent years, there have been developed and put in practical use various control technologies for vehicles, such as traction control, braking-force control, and torque-distribution control. These technologies generally utilize a road-surface friction coefficient for the calculation or correction of required control parameters. In order to perform the control properly, it is necessary to estimate a road-surface friction coefficient with high accuracy.
With regard to technologies for estimating a road-surface friction coefficient, the present applicant also has proposed in Japanese Unexamined Patent Application Publication No. 8-2274 a technology for estimating a road-surface friction coefficient from, for example, a steering angle, a vehicle speed, and a yaw rate using an adaptive control theory. According to this technology, a yaw movement or lateral movement of a vehicle is modeled, and is compared with a yaw movement or lateral movement of an actual vehicle. In accordance with this comparison, the tire characteristics are estimated every second so that a road-surface friction coefficient can be estimated.
However, in the above-mentioned device , there are cases where estimation of a road-surface friction coefficient is not possible such as when a steering angle is at 0°. In that case, even if the road condition has changed from a low μ road to a high μ road, the previously estimated value of the road-surface friction coefficient for the low μ road will still be used on the high μ road. This makes it difficult to perform vehicle behavior control with high accuracy. For example, in the case of traction control where torque is controlled by calculating an optimum grip force using an estimated value of a road-surface friction coefficient, an acceleration control amount that corresponds to a low μ road will be unfavorably set even though the vehicle is actually on a high μ road. In this case, there is a possibility that the vehicle will not exhibit its original acceleration performance.